1. Field of the Invention
The present invention relates to a security device including a security tag for fixing to a surface such that relative movement between the tag and the surface can be detected. The device may also include a monitoring circuit for monitoring an electrical characteristic of the tag such that an alarm is triggered when relative movement is detected.
2. Description of Related Art
In shops where valuable items are on display there is a need to prevent or inhibit theft of the items. A known security tag used for this purpose comprises a flexible substrate with an adhesive backing for bonding a head portion of the substrate to the surface of an item being protected: and an electrically conductive track defining a circuit between two locations on the tag, at least part of the conductive track being located between the substrate and the adhesive backing.
The tag has a release layer adjacent the conductive track so arranged that, when an attempt is made to peel the tag away from a surface to which the head portion is fixed, some portions of the track remain bonded to the substrate while others remain bonded to the adhesive layer. The circuit is thus disrupted and the change in resistance is used to trigger an alarm.
This device has several problems. For example, it is designed as a disposable item and is relatively expensive to manufacture due to the number of manufacturing processes. Moreover, the device has to be manufactured within strict electrical tolerances for its most commonly used application, and this results in a relatively high proportion of rejects during manufacture. The device also relies on the properties of the release layer (ink) which can alter during drying.